FIG. 1 and FIG. 2 illustrate examples of optical signal processing devices used in optical communication or optical measurements for improving a signal to noise (S/N) ratio of an optical signal itself, as disclosed respectively by B. K. Nayar, et al., “Concatenated all-optical loop mirror switches”, Journal of Modern Optics, Vol. 40, No. 12, pp. 2327–2332, 1993, and by K. E. Stubkjaer, et al., “Wavelength conversion devices and techniques”, Proceedings of 22nd European Conference on Optical Communication ECOC'96, ThB. 2. 1, 1996.
FIG. 1 is a diagram illustrating a device using a non-linear loop mirror (NOLM).
FIG. 2 is a diagram illustrating a device using a wavelength converter.
In FIG. 1, an input optical signal having a wavelength of λs passes through an optical coupler 1 and is input to a non-linear optical fiber 2. On the other hand, a continuum light beam having a wavelength of λprb is output from a laser 3. This continuum light beam passes through a polarization controller 4, and is input to an optical coupler 5, in which the continuum light beam is divided into two probe light beams. One of the two probe light beams passes through the optical coupler 1 and is input to the non-linear optical fiber 2, and the other probe light beam passes through a polarization controller 6 and is input to the non-linear optical fiber 2 from an opposite direction. In the non-linear optical fiber 2, by cross-phase modulation, a phase-modulated light beam is generated which has the wavelength of λprb but includes a signal component of the wavelength of λs. This light beam passes through the polarization controller 6 and interferes with a light beam in an opposite direction in the optical coupler 5, thereby, generating and outputting an intensity-modulated signal of the wavelength λprb with reduced noise.
In FIG. 2, a continuum light beam having a wavelength of λ2 is output from a laser 10, passes through a waveguide 11, and is input to an optical coupler 12. The input light beam is divided into two continuum light beams, and the two continuum light beams are respectively input to semiconductor optical amplifiers 13, 14. On the other hand, an input optical signal having a wavelength of λ1 is supplied from a waveguide 18. This input signal passes through an optical coupler 15, and is input to the semiconductor optical amplifier 13 in a direction opposite to the direction of the continuum light beam output from the semiconductor optical amplifier 13.
The optical signals respectively output from the semiconductor optical amplifiers 13 and 14 are mixed in the optical coupler 16, and a light beam is generated and output through a waveguide 17, which has a wavelength of λ2 and reduced noise due to interference and signal transcription caused by cross-phase modulation.
In addition, a method for improving an extinction ratio of an optical fiber is disclosed in Japanese Laid Open Patent Application No. 61-9606.
However, in the device using a non-linear loop mirror (NOLM) as illustrated in FIG. 1, because the non-linear optical fiber 2 is used, it is difficult to make the device compact. In addition, because it is required to set the power of the input light beams to be a sufficiently large value, it is difficult to reduce the power of the input light beams.
Further, in the device using a wavelength converter as illustrated in FIG. 2, when the frequency of the input signal is higher than 10 GHz, input/output performance of the device degrades, and it becomes difficult to output signals having the same wavelength obtained by converting the input signals, hence, such a device is not suitable for use of improving the S/N ratio of an optical signal.